Gordon Arnaut wrote:

Still, as you pointed out, the cure to torsional excitation is to dampen it,
not to build a stronger transmission. It should also be noted that one way
to avoid torsional vibration is not to run the engine continuously at the
rpm where excitation occurs (as with the non-counterweighted Lycoming).

No. The only cure for torsional excitation is the move the system's
harmonic into a range that will only be seen in low power operations and
then won't be used for very long. Tracy's PSRU moved the excitation
range down to between 500 and 800 rpm (working off the top of my head
here, so the numbers may be off). A well tuned rotary will idle in the
900-1000rpm range (prop at about 1/3rd of that).

Dampening doesn't exist. Elasticity in the system may shorten the
peaks, but you'll be left with a fatter mountain. The energy has to go
somewhere. The problem is that you'll still be hitting the shaft at
it's resonant frequency, causing a vibration in it. Each hit adds a
little to the vibration. If you repeatedly hit anything at it's
resonant frequency, each hit will add to the system vibration until it
either wears out very quickly or comes apart catastrophically. That
applies to reduction units, crankshaft, and control surface skins (yes,
flutter is a form of resonant vibration). Adding some elasticity to the
shaft may make the gearbox last longer, but you won't be able to do much
in those few seconds 8*)

Here's someone who says it much better than I:

http://rotaryaviation.com/PSRU%20Zen%20Part%202.html


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."